Incorrect Odometer Reading

[TFichter]

I have learned after taking my 1982 Spider on a 200 mile journey that the odometer reads more miles than I have actually traveled. When it should read 200 miles, it reads 213- about a 6% error. My wheels and tires are stock 185-60R 14’s. The speedometer is from an earlier model which had 13” wheels if that makes any difference. It seems to me that a bigger tire (185-65R 14) might correct the problem but I may be completely wrong. Any suggestions?

[DieselSpider]

The original tires are 165-82 13 which have a circumference of 74.30979788 inches and rotate 852.6466469 times per mile while a 185-60R 14 has a circumference of only 71.44031168 inches and rotates 886.8942269 times per mile.

Going to a 185-65R 14 would get you a circumference of 73.72847956 inches and a rotation of around 859.3694103 times per mile so you'd potentially be reading about 42 feet over for every mile traveled so that would be a lot closer than the roughly 212 feet over your getting with the 185-60R 14.

Have you done a GPS comparison on the speed difference too?

[TFichter]

Thank you for your knowledgeable reply. I will try a GPS measurement next!